The present invention generally relates to arc fault current interrupting (AFCI) circuit breakers. More specifically, the present invention relates to a temperature compensation circuit for an AFCI breaker.
AFCI circuit breakers are well known. These breakers comprise contacts that open upon sensing arcing from line to ground, or from line to neutral. Arc fault current breakers typically use a differential transformer to measure arcing from line to ground. Detecting arcing from line to neutral is accomplished by detecting rapid changes in load current by measuring voltage drop across a relatively constant resistance, usually a bi-metallic strip within the circuit breaker. Tripping of the AFCI breaker occurs when a predetermined number of arcs above a certain current level are detected within a predetermined time.
One characteristic of the bimetal element found in an AFCI breaker is that its resistance decreases as the ambient temperature decreases. It can be appreciated that the effect of the above characteristic is that the voltage developed across the bimetal element decreases with temperature for a given current. Typically, the electronic sensing circuitry in an arc fault current interrupting (AFCI) breaker is designed to have a fixed gain within a range of operating temperatures. Because the gain is fixed, low current arcs measured at low temperatures will develop a voltage that is interpreted to be below a certain threshold and will not be counted as xe2x80x9carcsxe2x80x9d. In other words, because the bimetal element caused a loss of gain as a result of temperature characteristics, the arcs or voltage developed across the bimetal element is made insufficient to count as an xe2x80x9carcxe2x80x9d. The result is that the number xe2x80x9carcsxe2x80x9d, or the number of voltages that are below a predetermined current threshold are reduced along with the sensitivity of the AFCI breaker to the occurrence of arc faults.
Schemes for an increase in overall gain are known. This approach typically solves problems related to inadequate gain at low temperatures. However, nuisance tripping is increased at room and higher temperatures. To offset the increased gain and prevent nuisance tripping, changes to decrease the response to low frequencies can be made. Arc faults typically cause a signal that is high frequency in nature, whereas noise is typically low frequency in nature. This approach works to reduce nuisance tripping, however, arc faults can cause a signal that has a low frequency. Therefore, changes to decrease the response to low frequencies can reduce the sensitivity of the circuit breaker to some arc faults. In sum, a trade off between gain, frequency response, and temperature is difficult to make.
In an embodiment of the present invention, a temperature compensation circuit is used for an arc fault current interrupting circuit breaker. The arc fault current interrupting circuit breaker is configured to stop a flow of electrical current to a portion of an electrical distribution circuit. The temperature compensation circuit includes a low pass filter configured to receive a signal indicative of a voltage in the electrical distribution circuit. The low pass filter includes an input resistance configured to receive the signal. The input resistance has a positive temperature coefficient. The low pass filter also includes an operational amplifier having a first input and an output, the first input is electrically connected to the input resistance. A feedback resistance is electrically connected to the first input and to an output of the operational amplifier, and a feedback capacitance electrically connected to the first input and to an output of the operational amplifier.
In an alternative embodiment, the feedback resistance has a negative temperature coefficient. In yet another embodiment, both the input resistance has a positive temperature coefficient, and the feedback resistance has a negative temperature coefficient.